A major effort will be on the molecular mechanism of transposition of transposon Tn3. In the movement of this DNA element from one site to another, the recombination is site specific for the transposon, since no adjoining DNA is transferred, but occurs almost without regard to sequence for the site of insertion. We wish to determine what enzymes are involved in the cutting out of the transposon and ligation to the recipient DNA. We will determine if our purified transposase has enzymatic activity. We hope to get transposition in vitro. Topoisomerases, by their very nature, break and rejoin DNA. In the reactions we had been studying, joining merely sealed transient breaks in a single DNA molecule. We now have evidence that topoisomerase can join to a DNA segment to another partner, i.e., recombined. We will study the mechanism of this reaction. In particular, we wish to isolate the intermediates and study the role, if any of DNA sequence homology.